Fix videos from being unrecognised by picture slideshow.

This allows videos to be played in a picture slideshow _if_ the user has video extensions enabled for their picture library.

Even though there is code to support videos, they do not play since slideshow items do not have a mimetype or VideoInfoTag. This results in a black screen when a video is encountered.

Issues that have also been resolved are:
- correct transitions to/from the original(previous) window - WINDOW_SLIDESHOW - WINDOW_FULLSCREEN_VIDEO, etc.
- issues of loading videos on top of each other, if a video is followed by a video
- preventing background loaded images from showing between two consecutive videos
- including videos in the list of items to be shown in a non-"slideshow" mode (i.e. just selecting a picture from a list)